Luke Coffey is the Margaret Thatcher Fellow at the Heritage Foundation, a Washington, DC-based think-tank. He previously served as a Special Adviser in the Ministry of Defence. Follow Luke on Twitter.

The Defence Secretary Philip Hammond’s announcement this week that Britain will be ordering reactors for a new class of nuclear submarines is good news for Britain’s future security, the Royal Navy and British jobs.

This announcement has, of course, revived debate surrounding the future of Britain’s nuclear deterrent. As ever, the Liberal Democrats’ have offered their favourite arguments in favour of so-called cheaper alternatives to the current Trident system. However, these arguments fall flat once they hit reality.

To be credible, any nuclear deterrent must be maintained continuously. Any adversary must know, or at least believe, that the UK has a nuclear strike capability beyond their reach and that it is always deployed and ready for use. This is the assumption that underpins any decision taken in the Ministry of Defence regarding the next generation of Britain’s nuclear deterrent. As military experts will tell you: if you do not have continuous deterrence, then you do not have any deterrence.

Let’s examine some of the alternatives being argued.

A land-based nuclear deterrent system would provide continuous coverage but would also create a static target for any adversary to hit in a first strike. It would be interesting to see which Members of Parliament are going to want this in their constituency. In order to achieve the same capability as the current Trident system, a land-based nuclear missile system would have to be spread across a distance one and a half times the size of Wales. Only at that scale could system survivability be assured in the event of a nuclear war. With the geographical vastness of the United States or Russia this is possible. In the United Kingdom, this is completely unrealistic.

Another alternative is to use a submarine-based nuclear deterrent equipped with nuclear-armed cruise missiles instead of Trident missiles. This is a particular favourite of the Liberal Democrats. This way, supporters argue, the new Astute Class attack submarines entering service could also be used for nuclear deterrence, thus saving the cost of developing an additional class of submarine. While this sounds nice in theory, it is unworkable in practice.

A new type of nuclear-armed cruise missile would have to be developed—something far more expensive than retaining the Trident missile. Under the current budgetary constraints of the MoD, this option is simply not economically feasible. Furthermore, cruise missiles have several major disadvantages:

a limited range--submarines attempting to deliver these missiles will need to come much closer to an adversary, leaving them far more vulnerable to detection,

subsonic speed—cruise missiles fly much slower, making it easier for foes to intercept them,

limited firepower—they can carry only one warhead per missile, as opposed to the 12 nuclear warheads each Trident missile can carry. You would need many more of the cruise missiles.

As for an airborne-delivery system, that would require the design and development of a new strategic bomber. Experts say at least 20 strategic bombers would be required to ensure a continuous level of deterrence—in this case multiple planes continuously in the air. The MoD has neither the money to procure a new strategic bomber nor the time to develop one before Trident’s out-of-service date. Furthermore, this would require a new dedicated fleet of air-to-air refuellers, at least two air bases, and the use of cruise missiles. The price tag quickly adds up.

Britain’s credible nuclear deterrent is a vital element of NATO’s collective security. NATO’s 2010 Strategic Concept stated that “the supreme guarantee of the security of the Allies is provided by the strategic nuclear forces of the Alliance.” As long as any regime from any part of the world poses a plausible nuclear threat against the West, NATO must remain a nuclear alliance and the UK must continue to provide its share of nuclear deterrence.

The world is getting more dangerous. As we have seen in North Korea, and as we will probably soon see in Iran, nuclear weapons are proliferating, not decreasing. Nobody can accurately predict the threats that the UK will face between 2028 and the 2060s – when the next generation of the deterrent will be in service—just as no one 25 years ago could have anticipated the collapse of the Soviet Union. Defence decisions must be taken in the long-term interest of the nation, not to score short term political points.

The Trident missile-bearing submarine fleet remains the most cost-effective way to deliver the level of nuclear deterrence Britain requires in the 21st century. This is why the current system needs to be updated and renewed. It is actually good value for money too. There is no problem spending £12 billion for three weeks of Olympic festivities so there should not be a problem spending £25 billion on 30 years of Britain’s future nuclear security. The Tories were right to overrule their Coalition partners on this matter of national security.